Active microwave couplers of the combiner or divider types as incorporated in MMICs used to suffer from the drawback of being relatively bulky and difficult to integrate.
A first improvement was obtained with active couplers, known as line-unified field-effect transistors (LUFETs) using field-effect transistors (FETs) having accesses unified with uniplanar interconnections. Such couplers are described in particular in the document entitled "Divider and combiner line-unified FETs as basic circuit function modules" published in September 1990 by T. TOKUMITSU et al., pp. 1210-1226, Vol. 38, No. 9, IEEE MTT.
Unifying accesses for the transistors makes it possible to take advantage of the slots formed by the metal strips constituting the electrodes of the transistors. It is thus possible to reduce the dimensions and the complexity involved in making a coupler. This also makes it possible to increase the frequency bandwidth of MMICs made in that way. Nevertheless, the resulting electrical performance, particularly in terms of insertion gain, remains limited. Furthermore, it is not always possible to integrate such a coupler directly in a more complete balanced circuit, and as a 180.degree. combiner in a balanced mixer, for example, since such a combiner requires inlet accesses that are floating and outlet accesses referenced to ground, which is not possible with the desired coupler.